1. Field of the Invention
This invention relates to radiant energy sensors, and more particularly, to a radiant energy sensor employing a damaged crystalline layer caused by excessive doping to trap or getter heavy metal impurities to enhance the majority carrier lifetime of the detector material.
2. Description of the Prior Art
Monolithic silicon charge coupled devices (CCD) type sensors which are currently being developed for high performance infrared detection and imaging systems utilize extrinsic indium or gallium doped silicon substrates. For many applications, extrinsic substrate material of extremely high quality is needed, for example, substrates of high or long carrier lifetimes are required for the high data rates utilized in forward looking infrared (FLIR) type scanned arrays. High carrier lifetimes are also important in the self-scanned staring sensors. This insures that for performance at very low background photon fluxes, the detector provides sufficient detector signal and consequent detector noise which exceeds the inherent CCD noise. Otherwise the detector would be limited by CCD noise.
It is widely recognized that the shallow acceptors, N.sub.A, such as boron, which are unavoidably present as residual impurities in highly doped P-extrinsic silicon, must be compensated as closely as possible by donor impurities, N.sub.D, such as phosphorus, in order to achieve high lifetime detector material. The use of neutron transmutation of silicon for introducing known amounts of phosphorus into extrinsic indium doped silicon to produce precision compensated infrared detector material has been developed and described in an article by R. N. Thomas, T. T. Braggins, H. M. Hobgood, and W. J. Takei, published in the Journal of Applied Physics, Vol. 49, page 2811 (1978). Extrinsic detector material with net (N.sub.D -N.sub.A) donor desnities as low as 2.times.10.sup.12 cm.sup.-3 have been achieved using neutron transmutation of silicon.
In the prior art a damaged crystalline layer has been formed by high concentrations of impurity atoms in silicon power and switching transistors to enhance the minority carrier lifetime in the material.
In the fabrication of infrared monolithic silicon focal plane arrays (MFPA), an extrinsic substrate is subjected to a variety of processing steps including thermal oxidation, P.sup.+ and N.sup.+ diffusions, epitaxial growth, silox and polysilicon depositions, etc., where temperatures as high as 1100.degree. C. are normally employed. The quality of an extrinsic silicon substrate is adversely affected by high temperature processing. Specifically, a very large increase in the net donor compensation of an indium doped silicon substrate was observed after CCD fabrication. The net donor density of the virgin substrate material was observed to increase from about 5.times.10.sup.12 cm.sup.-3 to 1.4.times.10.sup.14 cm.sup.-3 for the processed material. Similar large increases in the net donor density were observed in other Si:In substrates when these were subjected to a high single high temperature boron diffusion. For example, when a shallow boron diffusion was performed from a BBr.sub.3 source at 1100.degree. C. for 70 minutes into a wafer, an approximately 100 fold increase in the net donor density was observed. The data suggests that carrier lifetime, and therefore responsivity is drastically reduced by conventional high temperature silicon processing.
The adverse effects of high temperature processing on the quality of extrinsic silicon substrate has serious implications in monolithic silicon focal plane array (MFPA) fabrication technology. Certain lower temperature processes such as ion implantation with 600.degree.-800.degree. C. anneal, high pressure dielectric depositions, etc., can be utilized; however, temperatures of 1000.degree. to 1050.degree. C. are still required for silicon epitaxy. The high donor densities which are apparently introduced into the extrinsic substrate during high temperature processing are not well understood at present. The effects associated with the diffusion or redistribution of the conventional, slow diffusing donor impurities, such as phosphorus or arsenic, seem unlikely and are typically confined to depths of only a few micrometers from the surface. Similarly, possible oxygen donor activation, which occurs in crucible pulled silicon, is unknown in the low oxygen content, float zone silicon used while investigating this phenomena.
It is therefore desirable to restore infrared sensitivity in P-extrinsic detectors following high temperature processing.
It is further desirable to provide a damaged crystalline layer to trap or getter impurities or donor atoms from the extrinsic silicon material of the detector.
It is further desirable to provide a damaged crystalline layer by diffusing a high concentration of impurities such as 10.sup.19 cm.sup.-3.